The use of gamma-ray densitometers calibrated for use as hold-up meters, to measure flow and hold-up in pipelines is well known. Examples include Schlumberger™'s Vx technology and the FloWatcher™ Densitometer (FWD). Schlumberger's Vx technology uses low-energy gamma rays (PE) measurements. These measurements may encounter several difficulties in monitoring solids in mixed flow pipelines.
Low-energy gamma rays do not go through metallic walls of a sufficient thickness. Pipelines are generally made of steel. As a result, signal attenuation is very high leading for pipelines of typical diameters, which, in addition, contain a flow of high-density materials such as bitumen, water and particulates and a low gas fraction. This leads to a low count rate in the PE counting window.
Furthermore, bitumen contains significant quantities (typically 10-1,000 ppm) of elements such as vanadium (Z=23) and nickel (Z=28), which have an atomic number Z much higher than the atomic numbers found in hydrocarbon and water (H=1, C=6, O=8). Sulphur (Z=16) may also be present at percentage levels. The attenuation cross-section for low-energy photons is proportional to Z4. Therefore, small quantities of bitumen with uncertain content can strongly affect the measured PE attenuation. This is similar to the problem encountered in some surface monitoring applications, where inorganic scale in the flow-meter pipe can give large errors in the measured fluid PE cross-section due to the presence of higher-Z elements such as barium, sulphur and calcium.
Bitumen is viscous and sticky and may also adhere to the low-attenuation transmission windows required for PE measurement through a pipeline. This introduces further errors and loss of flux for a dual-energy measurement. The degree of adhesion may depend on both the bitumen properties and the window material.